AbstractThe cationic polymerization of a series of pentafulvenes with two exocyclic substituents yields highly reactive unsaturated polymers, which are soluble in nonpolar organic solvents. The preparative aspects as well as some properties of the polymers are discussed. Most polymers are very sensitive to oxygen. For instance poly(5‐isopropylidene‐3‐cyclopenten‐1,2‐ylene) (2a), which is obtained with molecular weights up to 350 000, consumes more than one mol‐equivalent of oxygen per rep